The present invention pertains to optical imagers and, more particularly to an improved charge-coupled optical imager having a self-aligned blooming control structure and a method of fabricating the blooming control structure.
A charge-coupled imaging device (hereinafter referred to as a CCD imager) may be thought of as a two-dimensional array of "Light Sensing Element". Each light sensing element corresponds to a potential well created within a semiconductor substrate along with its overlying insulation and electrodes. The array usually consists of rows of these light sensing elements, called channels, separated laterally from each other by heavily-doped barriers, called channel steps. When a scene is optically imaged onto this array, photogenerated charge is stored in each element in an amount proportional to the local light intensity of the scene. The optical information is thus converted into an electrical signal. The CCD electrode structure may then be used to sequentially clock the stored packets of charge to an output amplifier region which then senses the amount of charge in each packet.
CCD imaging technology is well known in the semiconductor art and structures suitable for operation with one, two, three, and four phase clock pulse sources have been described in the literature. Also well known is the existence of both frontside and backside illuminated imagers. The present invention concerns itself with a problem that is common to all CCD imager structures and thus pertains to each of them.
The problem results when the imaging array is illuminated by a scene in which certain regions are much brighter than others. In this case, the portions of the array receiving the intense radiation (which may be 10.sup.5 times the average scene intensity) will become overloaded. Specifically, the intense radiation impinging on a particular location in the array results in the generation of more signal charge than can be stored at that location. The excess charge tends to spread to the adjacent element or elements along the channel as well as into adjacent channels. This spreading of charge will manifest itself as "blooming" of the image which is read out of the array. Accordingly, "blooming" is defined as the spreading of the charge originally accumulated in a light sensing element in such a way as to interact with charge accumulated in adjacent light sensing elements. The region of intense radiation in the scene may therefore appear, when read out and subsequently reproduced, to occupy a much larger area than that occupied by the original.
The basic idea behind blooming control consists of providing an overflow drain for excess photogenerated carriers. This drain can consist of a reverse biased region doped to the polarity opposite from the semiconductor substrate. These overflow drains can then be placed between the transfer channels replacing the channel stop barriers and interconnected and accessed at the end of the device opposite from the serial register readout. A potential barrier can then be created, between each channel and drain, such that only excess charge in the channel, will spill into the overflow drain. One method of creating this potential barrier is disclosed in the Bell System Technical Journal, "Blooming Suppression in Charge-Coupled Area Imaging Devices," C. H. Sequin, 51, 1923-1926. The overflow threshold (or barrier) potential is established by a special threshold electrode. This electrode, which runs the full length and overlaps to each side of the overflow drain will set the potential barrier between the drain and channel when an external bias is placed on it. When set exactly at the threshold potential, the electrode will allow all excess carriers to spill into the overflow drain.
Inasmuch as blooming will begin to appear when the electrode bias is above threshold, this bias is usually held below threshold to provide a margin for error. It should be appreciated, however, that the well capacity (the total amount of charge that may be stored in one element) will be affected by the barrier height. As the barrier is lowered below threshold, carriers (in addition to excess) that were otherwise "storable," will spill into the overflow drain. The well capacity is thus effectively lowered.
An essential requirement of this type of blooming control is that the special threshold electrode be precisely aligned over the antiblooming drain. The width by which the electrode overhangs each side of the drain will affect the barrier height (partly as a result of the small geometries and partly as a result of the reverse bias on the drain). The tolerance required for this alignment is extremely small and is not compatible with present IC manufacturing alignment capabilities. As may be understood from the above discussion, any misalignment, however small, will result in degradation of the blooming control and/or non-uniform blooming control and well capacity.
Accordingly, an object of the present invention is to provide improved charge coupled imagers.
A further object of the present invention is to provide a charge-coupled imager having a split threshold electrode which is self-aligned to the overflow drain.
Another object of the present invention is to provide a method for fabricating a split electrode antiblooming structure for a charge-coupled imager.